Gas phototube circuit



March 4, 1958 J. H. RICHARDSON GAS PHOTOTUBE CIRCUIT Filed Dec. 29, 1955 THRESHOLD "0N C LIGHT FLUX l 0/AM. OFPUNCHED HOLE CURRENT DISTANCE ALONG TAPE 0/? Fig. 2

CORD

, I 6 "OFF" "ON" L/GHT FLUX Fig. 3

IIYVENTOR. James H R/chardsan United States Etcnt GAS PHGTGTUBE CIRCUIT James H. Richardson, Los Alamos, N. Mex., assignor to the United States of America as represented by the United States Atomic Energy Commission Application December 29, 1955, Serial No. 556,372

2 Claims. (Cl. 250219} The present invention relates to electronic circuits incorporating a light sensitive device to provide an output voltage of a binary nature and in particular to a sensitive phototube circuit in which the sensitivity of the phototube is variable according to the light level.

In certain commercial and scientific applications information stored on paper tape or punched cards is read out in a machine such as an electronic computer by passing the tape or cards between a light source and a phototube. The tape or cards normally block ofi the light passing from the light source to the phototube, except that when a hole comes into register, the phototube will see the light source and issue a responsive signal to an associated circuit.

Most tapes and punched cards are light in color so that writing or printing may be displayed on their surface and be readily seen. Usually they are lightly oiled in order to provide lubrication for the punch devices which record information on the tapes or cards. As a result, most tapes and cards are not opaque, but are translucent so that even when a hole is not in register some light will pass through to the phototube. To insure reliability it is desirable to have an electronic circuit which is not sensitively responsive to a light flux of a value less than a pre-selected threshold, and is sensitively responsive to all values of flux greater than the pro-selected threshold. In order to make the selection of the threshold level very positive it is desirable to use or arrange a photo sensitive element having the maximum possible light sensitivity at the reading level.

Photomultiplier tubes have a high sensitivity but cannot be used in most computers because of their large physical size. Cadmium sulphide photo cells do not ordinarily respond at a sufficiently rapid rate to changes of light flux. Photo-diodes and photo-transistors are too temperature sensitive to be used except in a carefully controlled environment. Therefore, a choice must be made between high vacuum phototubes and gas filled phototubes.

Gas filled phototubes are much more sensitive than high vacuum phototubes but at the usual operating voltages they have the disadvantage of an undesirably large and an inconsistent dark-current.

The present invention provides a novel circuit arrangement which permits a gas phototube to be used without large dark-currents when the light level is low, which also provides a very sharp adjustable threshold level and provides output stability without regard to light fluctuations below the threshold level. The present invention also provides a novel and reliable circuit for detecting the presence of holes in cards or tapes without ambiguous readings, and a circuit which is simpler and has fewer parts than circuits of the prior art, a substantial advantage in electronic computers.

It is therefore an object of the present invention to provide a novel electronic circuit incorporating a gas phototube suitable for use in punched tape and punched card readers.

Patented Mar. 4, 1958 ice Another object of the present invention is to provide a simple sensitive gas phototube circuit which is relatively insensitive to light flux below a certain chosen level.

Another object of the present invention is to provide binary (i. e. two level) output signals responsive to the intensity of the incident light according to whether the light is below or above a selected threshold level.

Further objects of the present invention will be apparent from the specification and from the drawings hereby made a part of the specification wherein:

Figure 1 is a schematic diagram of a preferred circuit according to the invention, and

Figure 2 is a graphical representation of the circuit output while a hole in a tape or a card is passing in front of the photo cell, and

Figure 3 is a graphical representation of the current through the photo cell in response to variations in the incident light flux.

By the novel arrangement of this invention a phototube is connected in an electronic circuit with an associated electronic device that operates normally in one stable state and is biased so that the expected low level light through the translucent tape or cards produces a current too small to change the operating conditions of the electronic device. However, when a hole in the tape or in the card begins to pass in front of the phototube and the light level subsequently reaches a pro-selected threshold, the phototube causes a change in the electronic device of the circuit so that it is at a second stable state. As a result of this change the voltage applied to the phototube is substantially increased causing a corresponding increase in its sensitivity and an increase in the current through the phototu'be. A light level greater or less than the selected threshold is detected for recording purposes by monitoring the voltage level at a point in the electronic circuit which changes as the phototu'be changes from one of its two operating conditions to the other.

With reference to Figure l, a preferred embodiment of the invention, the circuit comprises a phototube 10, a twin triode 13, a source of voltage 16 and resistors 11, i2, 14, 15 and 2%. Voltage source 16 may be, for example, a battery or equivalent providing volts on either side of ground 37. Adjustable taps 19 and 13 may be respectively connected to the control grid of section T-i of tube 13 thru resistor 14 and to the control grid of section T2 of tube 13. The anode of phototubc it is connected thru resistor 11 to the anode of T-Z and thru resistor 12 to the negative 150 volt terminal of voltage source 16. The anode of tube section T-Z is connected thru resistor H to the positive terminal of the high voltage source 16. The cathode of phototube it) is connected to the control grid of tube T-l. Output voltage E may be taken either from the cathode of tube 13 or from the anode or" tube section T-Z. In Figure l, the preferred embodiment, values of components are shown for the convenience of those wishing to practice the invention. The phototube is type 927 and the duo triode is type G. E. 5844. It is understood that these values are examples only and not to be considered limitations.

In typical operation, voltage taps 1S and 19 are set so that a bias of negative 12 volts is applied to the control grid of tube section T2 and negative 20 voits appiied to the control grid of tube section T-i. Under this condition, tube section T-2 conducts and T-i is cut off. Because of the potential drop across resistor 15 due to the current thru T-Z, the voltage across gas phototube it) is relatively low. Since the sensitivity of a gas phototube is low at low anode voltage, very little current is passed thru the tube at low light levels and the dark-current is low, since there is practically no ionization within the of phototuhe current vs. luniens is shown.

' inFigure 3 corresponds to the same le ers in Figure conducts, theincreasedcurrent thru cathode resistor causes the grid bias on T -Z-t be relatively more e and T 2 is cut ofi with a subsequentlarge rise in anode potential due to the cessation lcf current thru resistor 15. This caus sthe anosi gpot r i on PhO IDbQ 1 o i e onsidera y, e hs i itrrof pha -stubs 1h s reet yi creased, the current thruthe p otgtubc is; i sod and thebjiasof the control gridof tube section 1 -1 is greatly n rease Tubesection T 2. is t -afie mu -0t d voltage E isat a new higher level; Further increaseof illumination ca ses; hesepoudvol ege le el to in rcme y ann amount which; is prop rtional otheiu eas f illumination. It may be seen that the thresholdlevel for changes. inphototube, sensitivity may beset by judicious selection of thefbias voltage applied. to the control grids of'tube 13. 7

As the holejn thetape or-card passes by and the light intensity decreases, the voltage at thecontrol gridof tub seotionT-l will decrease and theoutputvoltageE will decrease correspondingly. When thelightlevel is ata new. threshold level substantially lower than the first,

tubesection T-1 will cut-01f and the current will instan:

itaneously transfer back to the tube sectionT-2 causing theoutput voltage to step. to its initial voltage level and reducing, the voltagedrop across resistor 15. Initial con-v ditions are restored. The second threshold is lower than thelfirst becauseofthe common cathode resistor coupling between T1 and T-2. i

In order to obtain a large voltageswing at the grid of 'tubesection T-l, a very large grid resistor 1-: isrequired. For the preferred embodiment, it has been found that 22 megohrns is a practical. value. With suchic. large grid resistor, care must be taken to reduce the grid current of T4 to a small value as conditions will be similar to the operation of an electrometcr tube. Thus the anode voltage and cathode current is kept iower than aliowed by tube ratings and the heater voltage is reducedfrom the suggested operating level. in the preferred embodiment theheatervoltageis reduced about 15%.

With reference to Figure 2, the variation of E with the movement of a hole in the tape or card across the face of the phototube is shown. a i

The hole diameter is represented by Gl-l andtlie voltage E by the line labeled as such. The heavy line represents the'light flux, which rises as the Whole passes by. Although the light flux level is rising from point G, the

level of E remains essentially constant funtilthe light.

level reaches the on threshold-at Cbecauseof the low sensitivity of the photot u'oe below threshold. At this time tube section T 2 is cut off andthe voltage E approxim s the light level curve CD2. Any undulation in curve 7 will not appear on the output as separate ulses, unless be lowindicating a low 7 "1,820,813, a v r .to the junction of said network first and second r'e 4 At flux level E, the off threshold is reached and the phototube'cu'rrent immediately falls to level F, ahdori re turn to A the initial conditioiis are restored.

7 Thus it can be seen from the foregoing that a binary system is achieved and that the circuit of this invention issues essentially two discrete voltage levels which define either a hole or no holecondition without uncertainty. 7

Therefore the light pas sing;thruthetranslucent card or tape does not appear as'a possible signal nor does a moderate amount of fuzeinthe punchedhole cause light'un. dulations of sutficient magnitude to make it appean'that several holes have'passedby instead ofone.

It i pparent ha hhsshhi l d. n han? s h. me l the circuit herein illustrated to achieve the function of the present invention without departing from the teachings herein. For example, voltage E, can be taken from the anode of tube section T-2 in which case the circuit Will he mewtlr-b naw a he h asslh i mifiPPW m t he ht fins mwilltead zn s she h t hese. a d. re Th s snns t nhsneuls 991. e h sitate re 0w m eqahcebr a sh.; hipsd= s ;e h ah T e cathode coupling'oi T to T-Zcould .beby diff er ent means also. Other switching means which willgiyeessentially the sameresults as tube}; couldbe used, Under certain conditions it will be desirable toput an anode r?- sistor in the anode circuit of IE1, dependjng upon he l a ppl dprsaamp a 1.000 hthl could be usedin thecircuitofiFigure l. Thusit is in:

to one end of an anode load resistor and the other end: of the anode load resistor being connected to thepositive terminal of .the source of potential, the anode of the secondtriode being directly connected to the positive term nal of the source of potential, the grid offi the fir'st triode being directly connected to a point a intermediate p te t a asaidQp t nt l ource," a lta e. i ids fi k' ons n r hd. schh esi o av n one end of ea h. being l ct ica y. onnect d o e her; this Otherendofone of thef resistor's beingconn' ted'to the junction between thefir'sft tnbe anode and the'anodel'oad resistor, andfthe other end of the second resistor: b'ein'g connectedjto the negative terminalfoffsaidfsource of potential, said gas phototube having anfanod ev cathode, sai'dfphototu'be anode being directly connected ist rs, said phototube' cathode being connecteddirectly to the second triode grid, and a grid leak" resistorconne'ct between the second triode grid and a selecteddn mediate point. of potential. on saidpbtentialgsgnrfbei 2. A device for providing 'a fir'st output voltagejinjreq spouse tea light intensity below a selected level an'd a second'outputvoltagejin response to a light intensity above ofi threshold, tube secton'T z begins to conduct and initial conditions are restored.

With reference toFigure 3, a graphical reg: I

The letters regards light level. Fr cmfr'toB in Figure 3, theglight level isb ebwthe on.th reshold.. B, the: on? threshold is reached and the phototub e current I anode 6f the first said selected level comprising} first j-and seco nd triode 'electron tubes, the cathodes of the triodes beingco necte'd together and to a common'cathodeload resistor,

music Whi s hh stihs i sheds st ls second v tnodewithasourcefof'anodepotential, {direct l connection Betw en} the control 3 grid (if the second and a source of bias potentialhavin'gavalue bovec a direct electrical connectionjbetween the-[an I first triode and the sourcejofjanoide 'pote" est t s ser c nedt dd qse her" and; h

triode and the neg'ative pt t source of anode potential, a gas phototube having an anode and a cathode, means electrically connecting the phototube anode to the junction between the said pair of resistors, means connecting the phototube cathode to the control grid of the second triode, and a grid leak resistor connecting the grid of the second triode to a bias source having a value below cut-01f whereby conduction of the phototube due to incident light raises the value of bias on the control grid of the second triode, and when the light intensity reaches a selected value the second triode starts to conduct thereby increasing the negative bias on the first triode and causing an increase of potential across the gas phototube and by such regenerative action causing 6 V v a swift transfer of conduction from the first tube to the second tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,573,405 Clark -n. Oct. 30, 1951 2514 227 Bordewieck et al Oct. 14, 1952 2,749,533 Daniels June 5, 1956 FOREIGN PATENTS 699,922 Great Britain Nov. 18, 1953 

